1. Field of the Invention
This invention relates generally to uncooled infrared detectors and focal plane arrays, and more specifically to a ferroelectric/pyroelectric infrared detector. The invention relates even more specifically to a ferroelectric/pyroelectric infrared detector comprising a colossal magneto-resistive electrode material.
2. Description of Related Art
Uncooled infrared thermal detectors have recently been developed into a large-size focal plane arrays (hereinafter xe2x80x9cFPAxe2x80x9d). For ferroelectric (xe2x80x9cFExe2x80x9d) and pyroelectric (xe2x80x9cPExe2x80x9d) infrared detectors, the detector structures are capacitors, in which the FE or PE (xe2x80x9cFE/PExe2x80x9d) thin film is disposed between top and bottom thin electrode layers.
In one type of detector, the FE/PE thin film is disposed between a reflective bottom electrode and a semi-transparent top electrode. In this type of detector, the resonance of the absorption occurs within the detector structure itself. In a second type of detector, the FE/PE thin film is disposed between a transparent bottom electrode and a semi-transparent top electrode. In this type of detector, the resonance of the absorption occurs in the cavity between the detector structure and the reflecting layer of the readout integrated circuit (xe2x80x9cROICxe2x80x9d).
Because the resonant cavities facilitate multi-pass absorption, the properties of the electrode are important to the FE/PE detector quality and performance. The electrode layer must be semi-transparent, so that part of the light can pass through to allow multi-pass absorption, and the sheet resistance must be within a specific range in order to maximize the infrared resonance absorption. Other important considerations include the value of the electrode material""s lattice constant, the crystal orientation of the material, and the chemical compatibility of the material with both the remainder of the detector structure and the processing steps.
While conventional electrode materials satisfy some of the aforementioned criteria for FE/PE detectors, no completely suitable electrode material has heretofore existed. For example, in a common practice, xe2x80x9cnormalxe2x80x9d metals, such as Pt, have been employed. Pt, however, not only is not semi-transparent, but is highly reflective. In addition, with Pt, the formation of helices at the deposition temperatures required for growth of the crystalline FE/PE films causes further complications for the growth of oriented films. Other conventional materials that have been studied for use as the electrode material are some metal oxides such as, La-Sr-Co-Oxide (xe2x80x9cLSCOxe2x80x9d).
Though not heretofore considered for use as electrode materials, the use of colossal magneto-resistive (xe2x80x9cCMRxe2x80x9d) materials for uncooled infrared detectors is described in Goyal et al., A., xe2x80x9cMaterial Characteristics of Perovskite Manganese Oxide Thin Films for Bolometric Applications,xe2x80x9d Applied Physics Letters, Vol. 71 (17) (Oct. 27, 1997), pp. 2535-2537. CMR materials demonstrate an exceptionally large change in resistance with temperature as they transition from a ferromagnetic to a non-ferromagnetic phase. The transition temperature can be adjusted through appropriate selection of materials and process conditions. For example, at the transition temperature, CMR materials exhibit a high temperature coefficient of resistance with adequate resistance for an ROIC impedance match. When the temperature increases to room temperature, however, the resistance of the CMR material is very small. The results have demonstrated the feasibility of growing CMR thin films on perovskite oxide material substrates such as LaAlO3 and SrTiO3 with a resultant temperature coefficient of resistance of greater than 7%.
CMR materials have a perovskite crystal structure with a square base. The lattice constant xe2x80x9cxcex1xe2x80x9d of the square base of a CMR material is approximately 3.8 to 3.9 xc3x85 depending on the material composition. As indicated above, CMR thin films have been successfully grown on perovskite oxide substrates such as LaAlO3 and SrTiO3, and exhibit a good crystal orientation and a high temperature coefficient of resistance. These perovskite oxide substrate materials are employed because of the correspondence of their crystal structure and lattice constant to those of CMR materials. For example, SrTiO3 has a cubic crystal structure with a lattice constant of 3.905 xc3x85, and LaAlO3 has a pseudo-cubic crystal structure with a lattice constant of 3.79 xc3x85. These properties facilitate the growth of a CMR material on LaAlO3 and SrTiO3 with a resultant high crystal orientation and quality.
A general need exists to provide an uncooled ferroelectric/pyroelectric infrared detector which includes a semi-transparent electrode material. An even more specific need exists to provide an electrode material that satisfies the aforementioned lattice constant, crystal orientation, and chemical compatibility requirements.
It is an object of the present invention to provide an uncooled ferroelectric/pyroelectric infrared detector which comprises a semi-transparent electrode material. It is a further object of the present invention to provide a semi-transparent electrode material of the requisite lattice constant value, crystal orientation, and chemical compatibility.
Accordingly, in a first preferred embodiment, the present invention advantageously relates to a ferroelectric/pyroelectric detector comprising a lattice matched substrate material and a colossal magneto-resistive electrode material.
By using either a rock salt structure material such as, for example, NaCl, LiF, NaF, KF, or KCl, or a solid solution of LaAlO3 and Sr2AlTaO6, as the substrate, a high quality epitaxial CMR material with a high temperature coefficient of resistance can be fabricated.
In a second preferred embodiment, the present invention relates to a ferroelectric/pyroelectric detector comprising a non-lattice matched substrate material, a colossal magneto-resistive template material, and a colossal magneto-resistive electrode material.
By employing a colossal magneto-resistive material as the electrode material, each of the aforementioned electrode material requirements is satisfied. Additional advantages associated with the present invention, however, are also realized. First, a CMR material provides the added benefit that through control of the composition, growth conditions, and film thickness, the sheet resistance of the material can be varied as needed in order to optimize the resonant structure. By varying the composition of a CMR material, the properties of the material can be modified so as to provide a resistance behavior that improves the performance of the uncooled infrared detector. Secondly, a CMR material demonstrates exceptionally large changes in resistance with temperature as it transitions from a ferromagnetic to a non-ferromagnetic phase. Because the transition temperature can be adjusted through appropriate selection of materials, composition, and process conditions, the CMR transition temperature can be xe2x80x9ctunedxe2x80x9d so as to be close to the operating temperature of the device.
Thirdly, because many of the most important FE/PE materials have the same perovskite structure and similar lattice constant as the CMR materials, use of a crystal oriented CMR material as an electrode and/or a template layer can ensure the crystal oriented growth of the FE/PE thin film.
Thus, by selecting an appropriate CMR material, the performance of the FE/PE detector can be improved over that associated with either xe2x80x9cnormalxe2x80x9d metal electrodes or metal oxide electrodes such as LSCO. Hence, CMR electrode materials provide significant advantages which can result in an improvement in the overall performance of the FE/PE infrared detector, and thus the FPA.